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Enzymes grouped in 6 major classes: (p. 643) 1. Oxidoreductases: Double-barreled name
catalyze the reduction or oxidation of a molecule. 2. Transferases:
catalyze the transfer of a group of atoms from one molecule to another. e.g. transfer of phosphate between ATP and sugar
3. Hydrolases: catalyze hydrolysis reactions & reverse rxns. e.g. hydrolysis of an ester.
4. Isomerases: Catalyze conversion of a molecule into isomer. The cis-trans conversion of maleate & fumarate
5. Lyases: Rxns which break a double bond & add a small molecule (water or NH3) & the reverse rxn: elimination to form a double bond.
(Not involving hydrolysis or oxidation)6. Ligases:
These enzymes catalyze reactions which make bonds to join together (ligate) smaller molecules to make larger ones. (Uses ATP energy)
Each class has more specific subclasses:1. Oxidoreductases:
oxidases and reductasesdehydrogenases (remove 2 H; make double bond)
2. Transferases: transaminases (amino group),
kinases (phosphate group)3. Hydrolases:
lipases, proteases, nucleases, carbohydrases, and phosphatases (break ester and amide bonds)
4. Isomerases:racemases (D to L or L to D forms)mutases (structural isomer )
5. Lyases: dehydratases (remove H2O from substrate)decarboxylases (remove CO2 from substrate)deaminases (remove NH3 from substrate)hydratases (add H2O to substrate)
6. Ligases: synthetases (hook 2 substrates together)carboxylases (add CO2 from substrate)
Substrate and active site on enzyme must fit. Some substrate molecules will fit / others will not, Some substrates will react and others will not = “specificity.”
Catalysts lower activation energy by:
1.Aligning substrate molecules at correct angle2.Holding substrate while rxn occurs (less E)
The intermediate product when the substrate and enzyme co-mingle= enzyme-substrate complex
There are two models for enzyme activity:1. Lock & Key2. Induced Fit
The active site of an enzyme is usually a
crevice formed from a protein’s
2o and 3o structural characteristics.
Usually is only a small portion of protein
structure.
May be multiple active sites
The lock-and-key model for enzyme activity: Only a substrate whose shape and chemical nature complement those of the
active site can interact with the enzyme.
enzyme-substrate complex
The induced-fit model for enzyme activity: The enzyme active site is not exactly complementary in shape to
substrate, but is flexible enough to adapt to the shape of the substrate.
Close enough for government work!
Enzyme-substrate complex is held weakly by intermolecular forces:
•Hydrogen Bonding•Salt bridges + electrostatic forces•Hydrophobic & hydrophilic regions•Van der Waals interactions
Enzyme action animation
Enzyme specificity levelsEnzymes can vary in their specificity:
Some active sites are very shape specific and bind to only one compound.Others are more generalized and focus on a similar structure found on various “related” molecules.
1. Absolute specificity2. Stereochemical specificity3. Group specificity4. Linkage specificity
Factors affecting Enzyme Activity
Four main factors affect the reaction rate:• Temperature changes• pH changes• Changes in substrate concentration• Changes in enzyme concentration
Effect of temperature on the rate of an
enzymatic reaction.
Why does heatfirst raise, then drop off
the reaction rate?
What bonds would break due to increased
molecular vibration?
How do thermophile bacteria survive?
Effect of pH changes on an enzyme’s turnover rate.
Why does low / high pH affect an enzyme?What bonds would
break and why?
What type of bonds would be found in enzymes found in
acidophile bacteria?
A graph showing in enzyme activity
with a in substrate concentration
(constant temp, pH, & enzyme concentration).
Enzyme activity remains constant
after a certain substrate concentration
is reached.
Why?You can only work so fast!
(Turnover #)
A graph showing the in reaction rate with a in enzyme concentration for
an enzymatic reaction. Temp, pH, and substrate concentration are constant.
Why does the graph not level off?
Adding more workers to the assembly line,
production speeds up!
3 Types of enzyme inhibition:
1. Competitive inhibition2. Noncompetitive inhibition3. Irreversible inhibition
•Molecule with similar structure to substrate•Competes for active site bonding•Blocks active site temporarily•Enzyme activity reduced / stopped•Reversible reaction
(weak interactions: H bonding, etc)Increasing concentration of substrate can overcome inhibition effect (out competes for active site)
Substrate blocked
Competitive inhibition
e.g. antihistamines vs histadine decarboxylation(which converts histadine to histaminehistamine)
Viagra: interferes with nitric oxide breakdown (NO is a vasodilator)
Noncompetitive inhibitor•Bonds to another site other than active site•Tertiary structure of enzyme altered•Affects shape of active site (conformation change)•Lowers or blocks substrate affinity for active site•Reversible
Increasing concentration of substrate does not overcome inhibition effect